Generators that are commonly used for wind turbines are asynchronous generators, whether with coiled rotor or caged rotor or salient pole synchronous motors or cylindrical rotor. However, for some time now, it seems that permanent magnet generators have been those most often used as they provide a series of advantages compared with others, such as the low maintenance required, and the reduced possibility of machine failure. Within what are termed as permanent magnet machines, we can distinguish between the generators with permanent magnets on the surface and those with embedded permanent magnets.
Surface magnet generators are comprised of rotor hubs which house the permanent magnets. Whether the rotor material itself is magnetic or not, the elements which make contact with the permanent magnets in this type of generator must be constructed of magnetic materials. These magnets are then stuck to the surface of the rotor ferrule and are fastened on with carbon fibre, fibre glass or nylon fibre materials.
In embedded magnet generators, the magnets are protected by the magnetic plate poles and by shims placed upon them in order to achieve complete robustness both from a mechanical point of view, as the magnets are completely protected, and from an electrical point of view, as the currents needed to demagnetise the magnets are greater than in the case of machines with magnets on the surface.
These characteristics mean that generators with embedded magnets are usually preferred to generators with surface magnets for applications which demand high rotation speeds or for applications where the machine must operate under extreme conditions.
Various proposals are known for preventing flux lines penetrating the rotor in embedded magnet electric machines, all of which have in common the insertion of a non magnetic material between the magnets and the internal part of the rotor.
U.S. Pat. No. 5,684,352 describes an embedded magnet rotor made up of various superimposed laminates, on each of said laminates there are both ferromagnetic zones and non magnetic zones placed in such as way that they prevent magnetic flux leakage to unwanted areas of the generator. Each of these laminates contains slots in which to embed the permanent magnets along with a hole in which the rotor shaft is inserted. The made are made in one part and with one material where the ferromagnetic and non magnetic zones are only differentiated by the crystalline structure of the material. The permanent magnets embedded in these sheets make contact by their poles with the ferromagnetic material, and make contact on the upper and lower sides with the non magnetic material. Thus preventing magnetic flux short-circuits. Once the permanent magnets are inserted, they are secured to the sheets using epoxy resin adhesive.
Applications US2003/0062792 and US2003/0062790 describe methods for making a permanent magnet rotor by sintering techniques, where the magnetic flux lines are redirected using the magnetic and non-conductive properties of the different parts of the rotor. In said rotors, the dispersion of the magnetic flux to the interior of the rotor shaft is avoided using a rotor ferrule of a non-magnetic material.
Application US2004/0212266 describes a rotor of one sole part where the permanent magnets and non conductive parts are embedded and are used to direct the magnetic flow towards the exterior.
Two important disadvantages of the aforementioned rotors are their high inertia levels and the high cost, for which this invention is intended to provide a solution.